1. Field of the Invention
The invention relates to an acoustic apparatus for producing audio signals.
2. Related Art of the Invention
Audio diffusion in vehicles today is intended to ensure the same level of quality in every seat. The aim is for all occupants to hear virtually the same audio signal at the same quality and at the same volume; in complex systems, this includes with a stereo or surround effect. Normally, however, commercial appliances today allow only coarse balancing of the volume between “right” and “left” and between “front” and “back”. Finer volume adjustment for each seat is not possible. Techniques are known which are aimed at separating the audio output so that people who are physically next to one another can be provided with quite different audio programs. Thus, by way of example, WO 01/08449 A1 describes a method for reproducing audio sound using ultrasound loudspeakers, where the audio signal to be reproduced is linked by amplitude modulation to a carrier signal in the ultrasound frequency range. Such techniques are used to focus the sound using a modulated ultrasound signal. In this context, the audible audio sound is produced along the ultrasound beam radiated at a very high level on account of the nonlinearity of the air and is added up in sharply focused form.
An option for producing audio directly in the headrest of a vehicle seat is demonstrated in patent application EP 1 077 156 A1, for example. In this case, the loudspeaker is integrated in the headrest. The sound emerges through an integrated resonator in the form of a horn, which adjoins the loudspeaker. Another option for producing audio in the headrest with directivity is described in the specification EP 1 077 583 A2. In this case, the directivity is produced by also making use of the sound transmitted toward the back of the headrest from the loudspeaker integrated in the headrest. The general benefit of audio diffusion as a near field in the head region is that the person in direct proximity is provided with good diffusion of sound and people a little further away are provided with a significantly reduced volume of diffused sound. In addition, the diffusion of sound at the headrest provides a good way of achieving spatial audio effects such as stereo. A drawback of these arrangements in which the acoustic system's loudspeaker is directly in the headrest is the reduced accident safety, in particular, since relatively large hard objects are integrated in the otherwise soft headrest and could result in head injuries. In addition, integrating relatively large-volume loudspeakers greatly restricts the visual design options for the headrest.
The specification JP 04 172 795 A describes an acoustic apparatus in which the sound transducer and the sound emergence location are physically separate from one another. In this context, the sound is routed between the sound transducer and the sound emergence location by a tubular sound line. For acoustic impedance matching of the air within the sound line to the environment, a horn-like line termination is proposed. In this case, the line termination is preferably created by means of suitable shaping of sound-absorbing material in order to damp reflections. The patent DE 689 19 495 T2 describes a similar system with sound transmission between the sound transducer and the sound emergence location, the acoustic impedance matching in this case being effected by placing ring elements at the sound emergence opening. In this case, the material from which these ring elements are made has a permeability which corresponds to that of air. In this context, the additional application of the ring elements increases the dimensions of the emergence openings of the sound line, however.